Heavy oils as an energy resource represents more than half world's oil resources, according to the International Energy Agency (World Energy Outlook 2008). In this regard, it is important to note that the American Petroleum Institute classifies heavy oil as having an API gravity between 10 and 22.3.
The demand for heavy oil has been marginal, due to high viscosity and complexity of its composition, which makes its production or recovery from the reservoir difficult. To meet the demand of light crude oil, which is currently in decline, increased production of heavy and extra-heavy oils could be carried out in several regions such as the Gulf of Mexico and North-eastern China. Table 1 shows the heavy oil reserves in Mexico (Annual Report 2009, Petróleos Mexicanos, Pemex), which shows that more than half of proven reserves in Mexico are of heavy oils.
TABLE 1Heavy oil reserves in Mexico (Annual Report, Pemex 2009)Reserve typeMillion barrelsHeavy oil (%)Proven10,404.261.3Probable10,375.852.1Possible10,149.849.8
Reservoir production starts when oil flow occurs naturally to the surface (primary recovery), and then is complemented by waterflooding or gas injection as secondary recovery processes. After these steps, tertiary or enhanced oil recovery (EOR) procedures are applied. The EOR process consists of injection of: miscible solvent, hydrocarbon gases or carbon dioxide, soda water, surfactants or soluble polymers, and biological products to reservoirs, and steam-assisted gravity. These processes include any methods to provide an energy source to the reservoir and to maximize the economic value of hydrocarbon reserves (Sen R., 2008: Biotechnology in petroleum recovery: The microbial EOR. Progress in Energy and Combustion Science. 34:714-724).
Primary recovery of heavy oils is reported as 10 to 15%, secondary recovery as 20-25% and enhanced oil recovery processes as 2-6% (Sandrea I. and Sandrea R., 2007: Global Oil Reserves—Recovery Factors Leave Vast Target for EOR Technologies. Oil & Gas Journal. Part 1, November 5 and Part 2, November 12; p 1-8).
Physicochemical EOR processes demand high energy consumption, which represents a high cost; and are not compatible with the environment. Therefore it is important to develop alternative technologies to improve heavy and extra heavy oil recovery from reservoirs. Some of these alternative technologies are related to microbial recovery methods, which represent a low-cost alternative and are environmentally compatible. Microorganisms produce a variety of products (gases, biosurfactants, biopolymers and solvents), which can reduce oil viscosity, change rock wettability, reduce interfacial tension and form stable oil-water emulsions; and also modify the oil properties, and thus increase the oil recovery in the reservoir (Bryant et al., 1998: Biotechnology for heavy oil recovery. 7th UNITAR International conference on heavy crude and tar sands).
In Mexican Patent Application No. MX/a/2009/014146, “Biotechnological process for hydrocarbon recovery in low permeability porous media”, filed Dec. 21, 2009, Olguín-Lora et al., refer a biotechnological process that increases recovery of oil with API gravity of 14 to 25 degrees, by stimulating the activity of extremophile indigenous microorganisms in the reservoir, capable of growing under anaerobic conditions at temperatures from 60 to 95° C., and pressures of 7 to 154.6 Kg/cm2 (100 to 2,200 psi). The described process can recover 11 to 30% oil, in addition to a secondary recovery process in porous media. This invention was not developed for recovery of oils with less than 14° API, temperatures below 60° C. and pressures greater than 154.6 Kg/cm2 (2,200 psi).
In Patent application WO 2009/009382 A3 “Process for enhanced oil recovery using a microbial consortium”, published on Jan. 15, 2009, Soni et al. describe a microbial method for recovering oil from naturally fractured sandstone or carbonate formations, with rock porosity >20% and reservoir temperature <90° C. This process is used for medium/light oils with viscosity <20 cp and API gravity >20°. During the process implementation, one stage is the shutdown of the well for a period of three weeks for the growth of microbial consortium, allowing microorganisms to release oil from the rock and increase recovery.
In Patent application WO 2009/001098 A3 “Method of enhancing oil recovery”, published on Dec. 3, 2008, Kotlar proposes a microbial oil recovery method, with heavy oil of 10 to 22° API. The method is based on microorganism injection isolated from the indigenous population of an oil reservoir, bitumen, or volcanic sludge. These microorganisms are incubated at temperatures from 70 to 100° C. The oil treated with microorganisms reduced the viscosity from 417 to 130 cp. This invention reached oil recovery up to 66% in columns packed with granulates, which are highly permeable systems.
In Patent application US 20070092930 A1 “Process for enhanced recovery of crude oil from oil wells using novel microbial consortium” published on Apr. 26, 2009, Lal et al. describe a process for oil recovery, using a mixed culture of thermophilic, acidogenic, barophiles and anaerobic bacteria, grown in a culture medium containing nutrients, minerals and a complex carbon source at temperatures from 70 to 90° C. Recovery examples of this invention were performed on columns packed with sand and residual saturation of 27.9%, obtaining an oil recovery of 8.9% due to microbial activity, but the examples do not show values for viscosity and API gravity of used oil.
In Patent application US 20070181300 A1 “System and method for preparing near-surface heavy oil for extraction using microbial degradation”, published on Aug. 9, 2007, Bushe and Rollins propose a heavy oil recovery system by using bacteria and fungi, with a nutrient addition. The field application is focused on hydrocarbons located near to reservoir surfaces, which means that the invention is used at relatively low temperatures and surface pressures. The authors describe only the recovery process, without providing information about oil and formation types, where the method can be applied.
Wankui et al., 2006: Microbe-enhanced oil recovery technology obtains huge success in low-permeability reservoirs in Daqing oilfield. SPE: Eastern Regional Meeting 2006, p. 45-52, 2006, describe the application on field of a biological process based on the injection of microorganisms in reservoirs. They report that injected microorganisms are adapted to reservoir conditions, degrade heavy oil, improve oil characteristics and produce biosurfactants, increasing oil production of 24.7 ton/d (before microorganism injection) to 40.8 ton/d. The authors did not provide information about viscosity and API gravity of oil recovered.
In the cited references, oil recovery using microorganisms has been carried out mainly in granular systems, sand-packed columns and, in some cases, low permeability porous media. Most of these references do not indicate the pressure condition to which they apply oil recovery processes; neither provides important characteristics of the oil such as viscosity and API gravity.
It is also important to note that the references described do not provide examples for conditions of greater than 2,200 psi pressure or recovery of heavy oil with API gravity <14 in consolidated systems such as cores; although there are recovery reports using microorganisms in columns, using heavy oils. These systems have higher permeability and porosity, and therefore are not comparable to consolidated systems. Thus, it is important to have processes that can be applied to carbonate and/or sandstone reservoirs and heavy oils to 10° API, where the nutrients and/or microorganisms are injected; and that include a well closure for a period of at least 7 days, less than reported in other patents.